1. Field of the Invention
The present invention relates to a machine part used as a magnet roll for developing, which constitutes a member of a developing machine for electrophotographic processes, such as a copier and a facsimile, and a laser printer as well. It also relates to a machine part used as a field magnet rotor of motors and the like. More particularly, the present invention relates to a machine part and a process for producing the same, said machine part made of a composite molding of a resin-bonded magnet which comprises a shaft having a resin-bonded magnet layer on the outer periphery thereof.
2. Description of the Prior Art
Magnet rolls are used in the development systems of electrophotographic apparatuses such as copiers and facsimiles, as devices for transferring toner particles to a photoreceptor. In a most prevailing electrophotographic system, a magnetic body comprising a metallic shaft which penetrates through said body is inserted into a sleeve in a non-contact manner. Thus, by rotating the sleeve relative to the magnet roll, the image having produced on the surface of the sleeve by magnetic adhesion of the toner particles is transferred to the photoreceptor without bringing the sleeve into direct contact with the photoreceptor. However, this type of development is now being replaced by a process which use no sleeves. This novel developing process uses a magnet roll which comprises a cylindrical or drum-shaped metallic shaft having an outer periphery covered with rubber magnets being arranged in a layer, and having further thereon metallic hemispherical floating electrodes with smooth surfaces. More recently, these types of magnet rolls are further improved by replacing the outermost layer of the floating electrodes with a magnet layer having a finely finished surface and made of rubber magnets that are fixed and adhered. In the electrophotographic process using such magnet rolls, the toner is directly adhered magnetically on the finely finished surface of the magnet rolls. This is the so-called direct-contact type electrophotographic development process. For example, JP-A-63-223675 (the term "JP-A-" as referred herein signifies an "unexamined published Japanese patent application") discloses a novel development apparatus of this type. The apparatus comprises a member which carries and transfers a developer containing a one-component magnetic toner to the developing area, from the vicinity of the latent image carrier comprising the photoreceptor. In the apparatus of this type, the toner is attracted on the surface of a magnetic body which is incorporated on the outer periphery of the transfer member, said toner being charged by frictional electrification between the charging member and the magnetic body to establish a thin toner layer on the surface of the magnetic body. The thin toner layer is then carried with the rotation of the magnetic body to transfer the toner to the photoreceptor.
In the magnet roll described above, the magnet layer which is provided around the shaft at a thickness of about 1 mm is made of a rubber-based magnet comprising a rubber based binder having dispersed therein isotropic barium ferrite grains. A hard blade, to which pressure is applied, is also established against the surface of said rubber-based magnet roll, to control the amount of the toner to be carried thereon. The magnet roll is manufactured by kneading a rubber material with ingredients such as ferrites to give a sheet, and after winding the sheet around the metallic shaft, the whole structure is subjected to press molding at a high temperature, which is then finished by polishing the surface.
Conventional magnet rolls and field magnet rotors for motors have been manufactured by applying pressure to adhere or to fit the magnet molding with shafts, spacers, etc. Because of the significant improvement in the performance of resin-bonded magnets using thermoplastic resins, the rubber magnets were replaced by the resin-bonded magnets to increase productivity. Thus, the resin-bonded magnets are now widely manufactured by insertion molding the shaft into the thermoplastic resin-bonded magnet.
However, the product still suffers an insufficient strength at the boundary of bonding between the shaft and the resin-bonded magnet molding; this is because, in general, the shaft or other metallic members have poor affinity with the resin-bonded magnet composition, and because there is generated a residual strain at the thermoplastic molding of the magnet. In the case of extrusion insertion molding, for example, the resin-bonded magnet molding undergoes complete separation from the shaft, and thus the total performance as a machine part is greatly impaired. Accordingly, attempts have been made to improve the bonding strength of the magnet body with the shaft. Such attempts include forming surface irregularities or cuttings on the shaft, or coating the surface of the shaft with a thermosetting adhesive based on an epoxy resin or the like and then heat-treating the whole structure after inserting the shaft into the magnet body to allow solidification of the adhesive. Those treatments, however, require extra costs and manpower, and yet, are not satisfactorily efficient. Moreover, long machine parts such as magnet rolls for use in developing steps of electrophotographic processes accompany difficulties in carrying out the adhesion process.
In the case of field magnet rotors for use in motors, a high strength against rotational fracture and a resistance against falling off of the shaft along the longitudinal direction are required to the resin-bonded magnet layer. However, sufficiently high values are not obtained as yet with respect to the two requirements above.
As described in the foregoing, the conventional composite moldings of magnets for use as machine parts, which is represented by rubber magnet rolls, have been suffering disadvantages summarized below, and it has been desired to overcome those problems.
(1) Cracks or openings form in the magnet layer or openings generate between the shaft and the magnet layer due to insufficient adhesion, during the cross-linking process for modifying the rubber magnets and the high temperature pressing of rubber magnets against the shaft for adhesion; and non-uniform structure also forms because of accidental local drop of pressure at the pressing, and such a heterogeneous structure leads to the generation of gas bubbles at the vulcanization or crosslinking; the phenomena above cause partial fluctuation in the properties of the magnet rolls, such as in the magnetic field intensity, etc., which results in a developed image having uneven density when transferred on a paper. In the case of direct contact type electrophotographic development in which the magnet roll itself is charged, the charged properties of the magnet roll are important. However, sometimes fluctuations in charged properties occur ascribed to the residual chemicals used at the vulcanization and crosslinking of rubber, or to the presence of other impurities.
(2) In addition to the high viscosity of the rubber itself, the incorporation of a filler such as a ferrite powder into the rubber further increases the viscosity of the rubber composition to make the processing more difficult. Because this tendency becomes more pronounced with decreasing the average diameter of the ferrite grains, a ferrite powder composed of grains with larger grain diameter may be used to ameliorate the processing properties, however, larger ferrite grains increase the surface roughness of the magnet rolls. To improve the surface roughness, a fine-grained ferrite powder (morphologically anisotropic ferrite grains suffice this requirement) should be used in the expense of lowering the processability and increasing the processing torque; thus, limits were imposed in the practical process.
(3) Because the rubber materials are incorporated as bulk materials, the ferrite powder cannot be uniformly dispersed in the rubber irrespective of the grain size. This leads to a magnet molding having a distribution in the concentration of ferrite. Such a distribution in concentration of ferrite impairs uniform magnetization of the product.
(4) Because a ferrite powder composed of grains having a relatively large average diameter is used, stable quality cannot be obtained for the magnet molding due to the lack of a fine surface as desired and to the incorporation of coarse grains. This results in a developed image suffering non-uniform appearance.
(5) Pinhole defects occasionally generate on the surface of the magnet layer. Such defects impair both the uniform magnetization and the formation of a uniform surface.
(6) Because the roll is manufactured by winding a rubber sheet around the shaft and then pressure molding the resulting structure, the seam of the rubber tend to be insufficiently fused. Such insufficient adhesion disturbs uniform magnetic and electric properties, and leads to the formation of irregularly developed images.
In the light of the circumstances above, the present inventors have found that the use of a thermoplastic resin in the place of rubber can circumvent the majority of the problems enumerated hereinbefore, and have proposed the use of a flexible composition for resin-bonded magnet based on ferrites, said composition comprising as the binder, a mixture of a chlorinated polyethylene with an olefin/vinyl ester copolymer comprising from 20 to 40 % by weight of vinyl ester and having a melt index of 50 or higher. Such a composition for a thermoplastic resin-bonded magnet is characterized by: that it has a sufficiently high mechanical strength despite an inorganic magnet powder being incorporated at a high concentration; that it is free of compositional migration and adhesion at the boundary between the magnet layer thereof and an object to be brought into contact with the magnet layer; and that it has a low melt viscosity at the hot melt molding, and yet it has favorable molding characteristics.
Thus was obtained a magnet roll having significantly improved properties as compared with the previous rubber magnet rolls; however, the problem of density unevenness and the like in the developed image still remained to be solved, and thus was looked for a further improvement. Particularly among the problems summarized above, the sixth problem which arise in connection to the presence of a seam in the magnet sheet, i.e., disturbance of uniform magnetic and electric properties, was found impossible to be solved with a prior art process. Thus, an improved process was desired.